Energy storage flywheel device

ABSTRACT

The energy storage flywheel device (6) includes a plurality of annular shaped composite sections (8) and an annular shaped insert (10). The insert (10) has a specific radial strength substantially greater than the composite sections&#39; specific radial strength. The composite sections (8) and the insert (10) are alternately stacked such that they have a common axis of rotation. They are bonded to each other such that shear stress is transferred between the composite sections (8) and insert (10). In some applications, the flywheel (6) may comprise at least one annular shaped metal matrix composite section (8).

This is a division of application Ser. No. 08/128,319 filed on Sep. 29,1993, now U.S. Pat. No. 5,452,625.

TECHNICAL FIELD

The present invention relates generally to a flywheel device and moreparticularly to an energy storage flywheel device having improved radialstrength.

BACKGROUND ART

The potential for using a flywheel as an energy storage medium is wellknown. From a materials technology standpoint, the primary factor thatcontrols the energy storage efficiency of a flywheel system is thestrength to density ratio of the material used for the flywheel.Conventionally, flywheels have been made of metals such as high strengthsteel. Steel, however, may not be a suitable material for a flywheelthat must store large amounts of energy efficiently. Specificdisadvantages of metallic flywheels include the high weight of theflywheel and the potential for dangerous fractures associated withrotating the flywheel at high speeds. In addition, conventional metalsare not well suited for high energy flywheels because their high densityresults in excessive loading during operation at high rim velocities.

Due to the disadvantages of metallic flywheels, wound fiber reinforcedresin matrix composite flywheels have been developed. Such flywheels,which have circumferentially-oriented fibers, have the potential forachieving high strength to density ratios in the direction of fiberreinforcement because of the availability of high strength, low densityfibers. Although resin matrix composites can achieve high hoop strength(strength parallel to the fiber direction), the mechanical properties ofthe composites are very anisotropic. For example, unidirectional resinmatrix composites may possess hoop strengths between about 1000 MPa toabout 2500 MPa, while the radial strength (strength perpendicular to thefiber direction) of these composites may be only between about 10 MPa toabout 20 MPa. This anisotropic behavior limits the use of resin matrixcomposites for flywheels. In many potential flywheel designs,particularly high speed flywheels, the flywheel's radial strength isexceeded prior to achieving full utilization of the inherent high hoopstrength of the flywheel. As a result fractures occur, making theflywheel unsuitable for many high speed applications.

Therefore, there exists a need for an energy storage flywheel devicecapable of achieving improved radial strength while maintainingsufficient hoop strength for operation at high rim velocities (e.g., 600meters per second or greater).

DISCLOSURE OF THE INVENTION

The present invention relates to an energy storage flywheel device thatachieves improved flywheel radial strength while maintaining sufficienthoop strength for operation at high speeds.

One aspect of the invention includes an energy storage flywheel devicehaving a flywheel that includes a plurality of annular shaped compositesections and annular shaped inserts with apertures filled with adhesiveor metal that bonds the composite section to the insert. The insertshave a specific radial strength substantially greater than the compositesections' specific radial strength. The composite sections and theinserts are alternately stacked such that they have a common axis ofrotation. They are bonded to each other such that shear stress istransferred between the composite sections and insert. In operation, theflywheel may be mounted on a shaft.

Another aspect of the invention includes a energy storage flywheeldevice having a flywheel that includes at least one annular shaped metalmatrix composite section.

These and other features and advantages of the present invention willbecome more apparent from the following description and accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of an energy storage flywheel device of thepresent invention.

FIG. 2 is a perspective view of another energy storage flywheel deviceof the present invention including an insert that has a wedge-shapedcross-section.

FIG. 3 is perspective view of another energy storage flywheel device ofthe present invention including interlocking features.

FIG. 4 is a perspective view of another energy storage flywheel deviceof the present invention further including a plurality of aperturesfilled with a material that bonds the composite sections to each otherand the insert.

BEST MODE FOR CARRYING OUT THE INVENTION

The energy storage flywheel device 6 of FIG. 1 achieves greater flywheelradial strength than prior art composite flywheels by alternatelystacking a plurality of annular shaped composite sections 8 with atleast one annular shaped insert 10. For example, as shown in FIG. 1,each insert 10 may be disposed between two composite sections 8.

The composite sections 8 may comprise any anisotropic material suitablefor flywheel fabrication such as wound fiber reinforced resin matrixcomposites or metal matrix composites. If the composite is a resinmatrix composite, thermosetting or thermoplastic resins are suitable forthe matrix. If the composite is a metal matrix composite, metals such asAl, Mg, or Ti may be used as the matrix. Preferably, the metal will beeither Al or Mg because of their relatively low densities.

The composite sections 8 also may comprise any conventional fiberscompatible with the matrix. For example, if the matrix is a resinmatrix, the fibers may be carbon fibers, glass fibers, aramid fibers, orany other fiber compatible with the matrix. If the matrix is a metalmatrix, the fibers may be graphite fibers, alumina fibers, siliconcarbide fibers, boron fibers, or any other fibers compatible with thematrix. If desired, the fibers may be coated to make them compatiblewith the matrix. The fibers may be oriented in the matrix in any waythat gives the flywheel 6 sufficient hoop strength for a desiredapplication. The composite sections 8 should be the major constituent ofthe flywheel 6 to provide sufficient hoop strength. For example, thecomposite sections 8 may make up about 75% to about 98% by volume of theflywheel 6. In some applications in which the composite sections 8comprise a metal matrix composite, the inserts 10 may be dispensed withentirely and the entire flywheel 6 may be made from the metal matrixcomposite because of the inherent higher radial strength provided by themetal matrix.

The insert 10 may comprise any material with a specific radial strengthsubstantially greater than the specific radial strength of the compositesections 8. For purposes of this application, substantially greatermeans at least 5 times as great. Specific strength is defined as theratio of the material's strength to its density. For example, the insert10 may be a monolithic metal, a metal matrix composite, or a resinmatrix composite. Preferably, the insert will comprise Al, an Al alloy,Ti, a Ti alloy, Ni, a Ni alloy, or steel. Most preferably, the insert 10will be a Ti alloy because of its high radial strength, low density, andlow to moderate cost. A small percentage of inserts 10 in the flywheel 6may increase the radial strength substantially, while only slightlydecreasing the hoop strength. For example, a flywheel 6 that comprisesabout 5% by volume to about 15% by volume of inserts 10 may be suitablefor many applications.

In a preferred embodiment, the specific radial elastic modulus of theinsert 10 will be greater than the specific radial elastic modulus ofthe composites sections 8, so that load transfers from the compositesections 8 in the radial direction to the insert 10. This preventsloading of the composite section 8 in the radial direction to the pointwhere failure can occur. Specific elastic modulus (or specificstiffness, another term for the same parameter) is defined as the ratioof the material's elastic modulus to its density. Similarly, thespecific hoop elastic modulus of the composite sections 8 should begreater than the specific hoop elastic modulus of the insert 10, so thatload is transferred from the insert 10 to the composite sections 8 inthe hoop direction. This prevents loading of the insert in the hoopdirection to the point where failure can occur. The composite sectionsalso should have a specific hoop strength substantially greater than thespecific hoop strength of the insert 10.

The composite sections 8 and insert 10 may be made with any suitableconventional techniques. After fabrication, the composite sections 8 andthe insert 10 are alternately stacked such that they have a common axisof rotation. Additionally, the composite sections 8 and insert 10 arebonded together such that shear stress and load are transferred betweenthe composite sections 8 and insert 10.

Any conventionally known method for bonding may be used to bond thecomposite sections 8 to the insert 10. For example, diffusion bonding,mechanical bonding, and adhesive bonding techniques may be employed.Similarly, a combination of any of the above techniques may be suitable.As shown in FIG. 2, an insert 10 with a wedge-shaped cross-section thatis thicker at its outer diameter than at its inner diameter may be usedto mechanically bond the insert 10 to the composite sections 8.

In another embodiment shown in FIG. 3, the insert 10 and compositesections 8 may be shaped to form interlocking features, such as teeth,that provide a mechanical bond between the insert 10 and compositesections 8. Adhesive disposed between the insert 10 and compositesections 8 also may be used to improve the bond between the interlockingteeth.

As shown in FIG. 4, the insert 10 may comprise a plurality of apertures12 filled with adhesive to bond the composite sections 8 to the insert10. Alternately, if the insert 10 is integrally east within a metalmatrix composite flywheel, these apertures can initially be leftunfilled to allow metal to flow between the composite sections 8. Afterfinal solidification of the metal matrix composite, metal that flowsthrough the apertures 12 will form an integral metal network that bondsthe composite sections 8 to each other and the insert 10.

The following example illustrates some advantages of the presentinvention without limiting the invention's broad scope.

EXAMPLE

A flywheel may be constructed by sandwiching a plurality of annularshaped inserts comprising an alloy of 6% Al, 4% V, balance Ti between aplurality of annular shaped wound fiber reinforced resin matrixcomposite sections. The composite sections may comprise 50 volumepercent (vol %) T800 graphite fiber (Amoco Performance Products, Inc.,Atlanta, Ga.) and 50 vol % epoxy matrix. The composite sections andinserts, which may be made with any of the conventional techniquesdescribed above, may be bonded together with any of the techniquesdescribed above such that the composite sections comprise about 95% byvolume of the flywheel and the inserts comprise about 5% by volume ofthe flywheel.

Table 1 shows properties of the flywheel materials, Ti-6Al-4V andT800/epoxy composite. Bonding annular sections of these materials in aflywheel construction such that shear stress is transferred between thecomposite sections and insert can increase radial strength significantlywhile only slightly decreasing hoop strength. As shown in Table 2, theaddition of the Ti-6Al-4V inserts may result in a 685% improvement inthe flywheel radial strength, with only a 3% decrease in the flywheelhoop strength. The overall volume percent and number of the Ti-6Al-4Vinserts could be tailored to meet specific design and fabricationrequirements.

                  TABLE 1                                                         ______________________________________                                                          Hoop     Radial  Hoop strength/                                     Density   strength strength                                                                              Density                                    Material                                                                              (g/cm.sup.3)                                                                            (MPa)    (MPa)   (MPa-cm.sup.3)/g)                          ______________________________________                                        T800/epoxy                                                                            1.58      2760     7       1746                                       composite                                                                     Ti-6Al-4V                                                                             4.44       966     966      218                                       insert                                                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                          Hoop     Radial  Hoop strength/                                     Density   strength strength                                                                              Density                                    Material                                                                              (g/cm.sup.3)                                                                            (MPa)    (MPa)   (MPa-cm.sup.3)/g)                          ______________________________________                                        5 vol % 1.72      2670     55      1553                                       Ti-6Al-4V                                                                     inserts/                                                                      95 vol %                                                                      T800/epoxy                                                                    composite                                                                     ______________________________________                                    

This improvement over the prior art offers significant design advantagesdue to the increased radial strength of the flywheel. Thus, the presentinvention includes a light weight, energy storage flywheel device havinggreater radial strength than prior art composite flywheels whilemaintaining sufficient hoop strength for operation at high speeds.

The invention is not limited to the particular embodiments shown anddescribed herein. Various changes and modifications may be made withoutdeparting from the spirit or scope of the claimed invention.

We claim:
 1. An energy storage flywheel device, comprising:(a) at leastone annular shaped composite section, and (b) at least one annularshaped insert having a specific radial strength substantially greaterthan the composite section's specific radial strength, wherein theinsert comprises a plurality of apertures filled with adhesive or metalthat adheres the composite section to the insert,wherein the compositesection and the insert are stacked along a common axis such that theyhave a common axis of rotation and are bonded to each other such thatradial load transfers between the composite section and insert.
 2. Thedevice of claim 1 wherein the insert has a specific radial elasticmodulus greater than the specific radial elastic modulus of thecomposite sections.
 3. The device of claim 1 wherein the compositesections have a specific hoop strength substantially greater than thespecific hoop strength of the insert.
 4. The device of claim 1 whereinthe composite sections comprise a metal matrix composite.
 5. The deviceof claim 4 wherein the metal matrix composite comprises a matrixselected from the group consisting of Mg, Al, and Ti.
 6. The device ofclaim 4 wherein the metal matrix composite comprises reinforcing fibersselected from the group consisting of graphite fibers, alumina fibers,silicon carbide fibers, and boron fibers.
 7. The device of claim 1wherein the composite sections comprise a resin matrix composite.
 8. Thedevice of claim 7 wherein the resin matrix composite comprisesreinforcing fibers selected from the group consisting of carbon fibers,glass fibers, and aramid fibers.
 9. The device of claim 1 wherein theinsert comprises a monolithic metal selected from the group consistingof Al, Al alloys, Ti, Ti alloys, Ni, Ni alloys, and steel.
 10. Thedevice of claim 1 wherein the insert comprises an alloy of 6% Al, 4% V,balance Ti and the composite sections comprise graphite fibers dispersedin an epoxy matrix.
 11. A device as in claim 1, wherein said filledapertures additionally bond composite sections to each other.
 12. Thedevice of claim 1 further comprising an adhesive disposed between theinsert and composite sections.
 13. The device of claim 1 wherein theinsert has a first side and a second side, and wherein the adhesivefurther adheres a first composite section bonded to the first side ofthe insert to a second composite section bonded to the second side ofthe insert.